Electro-optical subassemblies and method for assembly thereof

ABSTRACT

An electro-optical subassembly generally includes a base supporting at least one lead and a lens unit having a lens. The lead supports an electro-optical component. The lens unit is through transmission laser welded to the base such that the lens is aligned with the electro-optical component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of non-provisional application Ser. No.10/904,223 filed Oct. 29, 2004, which is incorporated by referenceherein in its entirety.

BACKGROUND OF THE INVENTION

Electro-Optical (EO) components, like single mode transmitter/receiveroptical sub-assemblies used in transmitters and receivers in fibercommunication, are usually packaged utilizing transistor outlineconstruction (sometimes referred to as a “TO can”). The EO componentsinside a TO can are wire-bonded to a number of leads that protrudethrough the package and allow signals to be routed to the EO components.These leads are bent and soldered onto a PCB board that contains theelectronic components and circuitry to drive the EO components.

A TO can has several disadvantages. The packaging process requiressignificant human labor with multiple alignment processes. Currentmethods are very much cottage industry like—often described a being akinto building a ship in a bottle. A large fraction of the cost of singlemode transmitter/receiver optical subassemblies is associated with thepackaging process.

The leads on known TO can structures are typically a few millimeters inlength and can cause a degradation of the frequency response of thesubassembly. The leads also have to be bent and soldered onto the PCBboard. This process is difficult to automate and is typically performedby hand. Yet another disadvantage is the mechanical tolerances stack up,e.g. the tolerance for the lens placement is affected by die placement.This requires that each component be positioned using a dedicatedthree—alignment system: one for die placement; one for lens placement;and one for the receptacle.

The present inventors have recognized a need for an electro-opticalsubassembly wherein the packaging process can be automated whileavoiding at least some of the disadvantages of known TO cans.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of a lead frame in accordance with anembodiment of the present invention.

FIG. 2 is an isometric view of a pre-molded lead frame in accordancewith an embodiment of the present invention.

FIG. 3 is an isometric view of a pre-molded lead frame in accordancewith an embodiment of the present invention.

FIG. 4 is an isometric view of a partial optical sub-assembly inaccordance with an embodiment of the present invention.

FIG. 5 is an isometric view of an optical sub-assembly in accordancewith an embodiment of the present invention.

FIG. 6 is an isometric view of an electro-optical subassembly inaccordance with an embodiment of the present invention.

FIG. 7 is an isometric view of an optical unit in accordance with anembodiment of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to the present invention, examplesof which are illustrated in the accompanying drawings, wherein likereference numerals refer to like elements throughout.

FIG. 1 is an isometric view of a lead frame in accordance with anembodiment of the present invention. Fabrication of an electro-opticalsubassembly 100 (see FIG. 5) starts with the stamping of a set of leadsfrom, for example, copper or tungsten copper. The central lead 10supports one or more optical electrical components, such as edgeemitting lasers, PIN detectors, Fabry Perot cavity lasers and VCSELs.The two flanking leads 12 and 14 may supply power or, depending on thedesired configuration, act as ground.

FIG. 2 is an isometric view of a pre-molded lead frame in accordancewith an embodiment of the present invention. A body 20 is molded overthe leads 10-14. The body 20 is generally circular and encompasses thecentral portion of the leads 10, 12, and 14. The body 20 may be providedseveral features to assist with alignment. For example a cavity 22 andprojections 24 and 26 may be used to assist with alignment of a laserwith a lens unit.

The body 20 may be molded from a variety of materials includingplastics. However, it may prove advantageous to use liquid crystalpolymer (“LCP”) for stability over a broad range of temperatures. LCP isa thermoplastic fiber with exceptional strength and rigidity (five timesthat of steel), and about 15 times the fatigue resistance of aramid.Very good impact resistance. LCP doesn't absorb moisture, has very lowstretch, doesn't creep like UHMW-PE fibre, and has excellent abrasion,wear, and chemical resistance. LCP's high melting-point (320 C) allowsthe retention of these properties over broad ranges of temperatures.

LCP has an unusual property of anisotropic coefficient of thermalexpansion due to a molecular structure comprising highly ordered linearchains which are melt oriented. The polymer chain undertake a regularorderly crystal like orientation during solidification in a mold, Bygating at the rear of the body 20, the flow direction can be aligned inthe Z-axis (parallel to the longitudinal axis of the leads 12 and 14) ofthe subassembly reducing expected drifts to an acceptable level based onthe low CTE of LCP (<5 ppm). The expansion in the X-Y directions shouldclosely match the expected expansion of a polymer lens (˜50ppm)—effectively minimizing movement due to temperature shifts about theoptical axis of the lens.

FIG. 3 is an isometric view of a pre-molded lead frame in accordancewith an embodiment of the present invention. FIG. 3 illustrates theplacement of an edge-emitting laser 30 and a PIN monitor 32 on thecentral leads 10. The laser 30 and monitor 32 may be electricallyconnected to the leads 12 and 14 with wire bonding.

FIG. 4 is an isometric view of a partial optical sub-assembly inaccordance with an embodiment of the present invention. A spacer in theform of a molded hollow cylinder 40 is formed and inserted into thegroove 22 of the body 20. A cavity 42 of the cylinder 40 surrounds theportion of the leads 10, 12, and 14 extending from the face of the body20 along the Z-axis thereby protecting the laser 30 and monitor 32. Thelength of the cylinder 40 is determined by the optical properties of thelaser 30, monitor 32 and the lens used in the electro-opticalsubassembly 100 (see element 52 in FIG. 5).

As with the body 20, the cylinder 40 may be molded from a variety ofmaterials including plastics. However, it may prove advantageous to useLCP. Further, it may prove advantageous to mold the spacer/cylinder 40as part of the body 20 or the lens unit 50 (see FIG. 5).

FIG. 5 is an isometric view of an optical sub-assembly in accordancewith an embodiment of the present invention. To complete theelectro-optical subassembly 100, a lens unit 50 is attached to thecylinder 40. The lens unit is provided with a lens 52, such as anaspherical lens. The lens unit 52 is generally shaped like a cap andslides over the cylinder 40. By closely controlling the surfaces of thebody 20, the cylinder 40 and the lens unit 50, the laser 30 and the lens52 can be precisely aligned within acceptable tolerances (in thesub-micron range).

It may prove beneficial to mold the lens unit 50 using a polymer. Theuse of polymer, as opposed to traditional glass, permits the formationof intricate 3-D geometry for registration purposes as well as theability to couple more light. Further, polymer lens fend themselves tomass production - reducing the overall cost of the electro-opticalsubassembly 100.

The cylinder 40 may be affixed to the base 20 and the lens unit 50 usingepoxy. However, it should be noted that the use of epoxy may increasethe possibility of drift (especially during cure) and may provedifficult to apply to the small parts comprising the electro-opticalsubassembly 100. For example, the overall length of the electro-opticalsubassembly 100 may be smaller than 6 mm while the diameter of thecylinder 40 may be smaller than 5 mm.

A more suitable joining technique is through transmission laser welding(TTLW). TTLW, well known method for joining two thermoplastics part, isundergoing a renaissance with the introduction of IR absorbing dyesallowing clear-to-clear polymer transmission welding. Older TTLWtechniques required that the first part to be joined had to be opticallytransparent and the second part had to absorb the laser energy. With thenew IR absorbing dyes, the second part can also be opticallytransparent.

In TTLW, a laser passes though the optically clear part impinging on thesecond part with the IR absorbing dye. The second part absorbs the lasercreating heat that in turn leads to plastification. The resultant localincrease in volume of the second (absorbent) part causes a surfacecontact with the first part (translucent) that causes plastification ofthe second part creating the weld. With the use of appropriate jigs,movement between the two parts during the welding process may beminimized. Further, as the heat is localized to the joint, the partsexperience little or no heat based distortion. The strength of the jointis quite high and may exceed that of the individual parts.

In the present invention, the second part may comprise the cylinder 40which can be doped with IR absorbing dye. The first part could be one orboth of the body 20 and the lens unit 50. TTLW techniques and apparatusare well suited for the cylindrical shape of the joints between the base20, the cylinder 40 and the lens unit 50 such that it is possible tocreate a hermitically sealed electro-optical subassembly 100. Further,as TTLW operations are suitable for large batch operations, themanufacturing of the electra-optical subassembly 100 can be automated toa level similar to that found in the microelectronic industry.

It will be appreciated by those skilled in the art that changes may bemade in the described embodiments without departing from the principlesand spirit of the invention, the scope of which is defined in the claimsand their equivalents. For example, while the base 20, cylinder 40 andlens unit 50 have been described as being cylindrical, different shapesand configurations may prove beneficial.

FIG. 6 is an isometric view of an alternative configuration which mayprove beneficial. The electro-optical assemble 600 generally comprises abase 610, an optical unit 620, The base 610 generally comprises acollection of leads 612 partially encased by a wedge shaped molded body614. Electro-optical components, such as a laser 616 and a PIN detector618, are fixed to one or more leads 612 n. The optical unit 620 has awedge shaped cavity 622 that accepts the base 610 and facilitatesalignment of the electro-optical components (such as 616 and 618) withan optical lens (not visible) formed as part of the optical unit 620.The electro-optical subassembly 610 mates with a port 630 thatfacilitates alignment of the optical unit 620 with an optical cable (notshown). The optical unit 620 and the base 610 are joined using TTLW.

The electro-optical assembly 600 provides many advantageous. The body614 has two wedge shaped arms defining a central opening for holding theoptical components. The leads 612 n can be formed using standardtechnologies and, if desired, can be configured to facilitate surfacemounting the electro-optical subassembly 600 onto a PCB board (notshown). The design of the base 610 allows the overall size of theelectro-optical subassembly 600 to be reduced as compared to a TO-can.This size reduction minimizes disruptive thermal expansions and reducesthe distance between the electro-optical components and the opticallens. Further, as the leads 612 n are anchored into the modeled body614, overall rigidity is increased. The emitting surface of the laser616 can be accurately positioned relative to the optical lens makingZ-alignment of the port 630 redundant. Since the optical lens and thelaser 616 are referenced against the same base, XY-alignment of the lensmay be redundant, reducing the typical three-alignment process to atwo-alignment process.

FIG. 7 is an isometric view of an optical unit 620 in accordance with anembodiment of the present invention. The optical unit 620 generallycomprises a body portion 702 and a lens 704. The body portion 702generally comprises a frustum having two opposing flat surfaces 708 aand 708 b. The opposing flat surfaces 708 a and 708 b may be molded orground into the body and serve as alignment features. The lens 704 maycomprise an aspherical lens. The exact configuration of the lens 704will be determined by the required function, for example coupling thelight from a laser with an optical fiber and/or coupling the light fromthe optical fiber to the PIN detector. The lens 704 may be molded withthe body 702 and then provided with a clear optical surface.Alternatively, the lens 704 may be turned after the body 702 has beenmolded.

1. A method for fabricating an electro-optical subassembly, the methodcomprising: forming a base including leads for supporting anelectro-optical component; attaching an electro-optical component to alead; forming a lens; and fixing the lens in relationship to the baseusing through transmission laser welding.
 2. A method, as set forth inclaim 1, wherein the step of forming a base comprises: forming theleads; and molding a liquid crystalline polymer body around the leads.3. A method, as set forth in claim 1, wherein the step of forming a lenscomprises: molding the lens using a polymer.
 4. A method, as set forthin claim 1, wherein the step of forming a lens comprises: molding thelens using a liquid crystalline polymer.
 5. A method, as set forth inclaim 1, wherein the step of forming a lens comprises: molding a lensunit comprising a body having a cavity at a first end in communicationwith a lens, the lens being formed in a second end of the body.
 6. Amethod for fabricating an electro-optical subassembly, as set forth inclaim 1, wherein the step of attaching the lens to the base comprises:forming a spacer; through transmission laser welding the lens to a firstend of the spacer; through transmission laser welding the base to asecond end of the spacer.
 7. A method, as set forth in claim 6, whereinthe step of forming a spacer comprises forming a hollow cylinder usingliquid crystalline polymer.